Traditionally, the inks used on digital and stencil duplicators are of the water-in-oil emulsion type. The continued use of such inks in the duplicating field derives from their possessing a number of important properties.
In particular, water-in-oil emulsion inks can be formulated to have rheological characteristics suitable for use in rotary printing machines, making it possible to control the amount of ink used in the printing process and the degree of ink penetration into the printing substrate, and to prevent "tail flooding", the effect whereby ink exudes from under the trailing edge of the stencil master on a single drum duplicator. Also, and of crucial importance, emulsion inks can be formulated to be non film-forming, which allows their retention within the printing drum of a duplicator during periods between use without film formation, skinning, or crystallisation, which would result in irreversible blocking of the mesh screen on the drum.
However, there are major disadvantages associated with the use of emulsion inks which limit the ability of current duplicating processes to compete with other imaging technologies. In particular, the degree of image-fixing that can be achieved in modern duplicating processes, which are relatively high speed printing processes typically printing up to 150 pages per minute, using an emulsion ink is limited. It can be appreciated that since emulsion inks do not solidify after printing, their continued liquid state on the printed sheet leads to the problems of set off and smudging. These problems become more serious when the substrate being printed is non-absorbent, rendering emulsion inks generally unsuitable for the printing of non-absorbent substrates.
Another problem with the use of emulsion inks in duplicating processes is that they achieve lower image density than other digital imaging processes, such as laser printing. This inferior image density is believed to result from the fact that in order to achieve image-fixing of an emulsion ink a certain proportion of that ink must be absorbed into the print medium, thereby detracting from image density.
Yet another problem with the use of emulsion inks in duplicating processes is that if left in the drum of a duplicator between periods of use the water content of the ink tends to be reduced by evaporation. As a consequence, print quality on start-up is less than optimum until the drum is purged by fresh ink from the supply container, or cartridge. Typically, therefore, prints on start-up are characterised by excessive ink usage accompanied by abnormally high levels of set off and smudging.
Yet another problem relating to the use of emulsion inks is the phenomenon of paper cockling which results from the absorption of water from the Ink into the fibres of the printed paper. This is particularly evident when a large quantity of ink is used, either to print a large surface area or to achieve a high print density. Paper cockle is not only unaesthetic, but also the high points of the distorted paper surface exacerbate set-off.
Since the introduction of the digital duplicating process, attempts have been made to address the above-identified shortcomings. However, most or all of the proposed improvements have retained the use of an emulsion ink. For instance, JP-A-06128516 discloses the use of a water-in-oil emulsion ink containing a hot-melt component, so that image-fixing can be enhanced after printing by the application of heat. However, the other disadvantages associated with emulsion inks described above still remain unresolved.
Further, the necessity to remove water during the drying/fixing stage of the process disclosed in JP-A-06128516 may impose limitations on the process, for instance in terms of print speed. In an Example given in that document prints are fixed by passing over a heated roll at a speed of 3 m/min, which is a significantly lower printing speed than the 18 -50 m/min range typical of modern digital duplicators.